The present invention relates to a gas flow measuring apparatus useful for measuring the flow of intake air of an engine or the like.
In a fuel injection type engine, the flow rate of intake air of the engine is measured and the fuel commensurate with the flow rate is supplied by injection to the engine by way of a fuel injection valve. As an intake air flow measuring system which is applied to such a type of engine, there is known a system which comprises a measuring tube in an engine intake manifold, which measuring tube in turn contains a pair of temperature-dependent resistance wires and a resistance wire for electric heater made of platinum, and the output signals of these devices are used to measure the flow rate of intake air (i.e., the gas to be measured). Such an apparatus has essentially an advantage that the flow rate can be measured accurately with a compact and simple construction.
In the prior art apparatuses of this type, the resistance wire for the electric heater and the first and second temperature-dependent resistance wires are made of a very thin platinum wire independently suspended in the flow of intake air on separate supporters.
This construction is such that when the platinum resistance wire is subjected to a shock due to a backfiring (which is a phenomenon caused when the ignition timing or operation timing of the intake or exhaust valve of the engine combustion system is incorrectly adjusted and the flame in the combustion chamber causes an explosive combustion of the fuel and air in the intake manifold), the very thin platinum resistance wire is likely to be easily broken.
Further, in view of the fact that the control starts for the purpose of measurement only after the temperature-dependent resistance wires have received heat from the resistance wire of the electric heater, the heat of the electric heater is required to be quickely transmitted to the temperature-dependent resistance wires for an improved response. Since the electric heater resistance wire and the first and second temperature-dependent resistance wires are mounted on separate supporters as described above, however, it is difficult to arrange the electric heater resistance wire and the first temperature-dependent resistance wire receiving heat from the former in such proximity to each other as desired, thus making impossible rapid heat transmission from the electric heater resistance wire to the first temperature-dependent resistance wire. Furthermore, a part of heat of the electric heater resistance wire is caught by the supporter thereof, thereby failing to be transmitted to the first temperature-dependent resistance wire. This results in a thermal loss and a proportionate error in flow measurement, thus deteriorating the measurement accuracy.